A central problem of neuroscience is to understand the mechanisms of behavioral recovery following neural injury. Recent preliminary findings from our laboratories, working jointly, provide the first demonstration of the restoration of non-associative learning by nerve regeneration. Thus we propose to use the leech as a model system to study the cellular basis of functional recovery following injury. This entirely collaborative work is the basis for the proposed cellular and behavioral studies on recovery of one form of learning, sensitization, following damage to central circuits. The project represents the convergence of our separate behavioral and synapse regeneration studies in the leech, combining in depth work on modulation of defensive shortening from one laboratory and work on the S interneuron, its connections and its responses to injury from the other laboratory. The proposed experiments will provide insight into the questions, (1) by what cellular mechanisms does the nervous system recover from injury and (2) what cellular changes underlie learning, including non-associative learning. The leech is particularly advantageous for studies of this type because its ganglia contain identifiable neurons capable of regenerating specific connections following axotomy, because individual neurons can be selectively ablated without injury to surrounding structures, and because stereotyped adult behaviors can be examined both in vivo and in vitro while recording from identified neurons, including the S cell. The proposed experiments will determine (1) whether modulation of defensive shortening eliminated by S cell ablation eventually returns, (2) whether S-cell axotomy is equivalent to ablation, which would indicate that simply breaking the chain of interneurons disrupts non-associative learning, (3) whether S-cell axon and synapse regeneration restores sensitization of shortening, (4) whether serotonin depletion resembles S-cell ablation because common pathways are disrupted, and (5) how the S cell is incorporated into circuitry that modulates behavior. We will focus on sensitization, which is eliminated by S-cell ablation, rather than on dishabituation, which is only impaired. The methodology will include electrophysiological recording, intracellular injection of morphological markers and proteases, laser microbeam axotomy, behavioral testing of leeches including semi-intact preparations, and confocal and electron microscopy. The proposed studies will reveal cellular circuitry for non-associative learning and basic mechanisms for restoration of plastic properties of the nervous system following injury.